A fuel cell system, which may be arranged in a motor vehicle as the only electric power supply or as an additional electric power supply, usually comprises a fuel cell, which is formed, as a rule, by a stack of individual fuel cell elements and which reacts an anode waste gas with a cathode waste gas to generate electric current. A residual gas burner, which reacts the waste gases of the fuel cell, i.e., anode waste gas and cathode waste gas while burner waste gas is formed, may be arranged downstream of the fuel cell. To generate a reformate gas, which can be fed as anode gas to the fuel cell, the fuel cell system may be equipped, in addition, with a reformer. The fuel cell operates exothermally in the state in which it operates at normal rating. The reformer likewise operates exothermally during operation at normal rating when it is operating with partial catalytic oxidation of the fuel.
Heat must be supplied to the fuel cell during a cold start of the fuel cell system, during which the individual components are started up from the ambient temperature, in order for the fuel cell to reach its operating temperature. Heat must likewise be supplied for a reformer operating with a catalytic converter in order for the reformer to reach its operating temperature. The fuel cell system has a poor energy efficiency during this start-up process. It will then likewise have comparatively high pollutant emission levels. To make it possible to keep the start-up operation as short as possible in terms of time, it is possible to design the residual gas burner and the reformer for the start-up operation such that the residual gas burner and the reformer generate as much heat as possible during the cold start operation, which can then be used to heat up the components of the system. However, such a design for the start-up operation inevitably leads to incorrect design or overdimensioning for the state of operation at normal rating. This leads especially to a reduction of the energy efficiency of the fuel cell system for operation at normal rating.